Blake, Roof of the Fo^irtJi Vcntncle. 103 



The wall of the pocket may be more or less fused to the 

 roots of the glossopharyngeus and vagus. The existence of this 

 fusion has given rise to the contention that the foramina here 

 are artifacts. Microscopical sections, however, effectually 

 demonstrate their presence. 



The roof the lateral recess caudal to the involution of the 

 choroid plexus along the margin of the posterior medullary 

 velum, consists in all mammals, when it exists, of a simple layer 

 of epithelium unsupported by nervous matter. Figs. 15 and 32, 

 In the lower mammalia there is usually a slight extension of the 

 epithelium of the roof beyond the choroid plexus forming a lat- 

 eral wall for the recess, Figs. 15 and 16, which is closely ap- 

 plied to the dura, and which closes in the cephalic portion of 

 the recess. Cannieu evidently observed this arrangement and 

 thus led to the conclusion that the recess is completely closed 

 in all mammals. 



While his observations are correct for the cephalic portion 

 of the recess, I have found in embryos of cat, dog, pig, sheep 

 and rat in their later stages, a complete disappearance of the 

 caudal portion of the recess, and thus a free communication 

 with the subarachnoid cavity. In man there is as a rule no ex- 

 tension of the roof epithelium beyond the choroid plexus in the 

 lateral portion of the recess, thus the dorsal margin of the fora- 

 men of Luschka is formed by the choroid plexus. Figs. 

 29, 32 et. seq. 



It is out of the province of this paper to enter into a dis- 

 cussion at length of reasons for and against the division of the 

 third cerebral vesicle into two cephalic segments. 



Several of the arguments of those advocating the division 

 have been, it seems to me, successfully refuted by Spitzka 

 and others. 



If we divide this portion of the brain into epencephalon and 

 metencephalon there necessarily follows a division of the cavity 

 of the fourth ventricle into epicoelia and metacoelia ; and fur- 

 thermore, if we cannot make this division of the cavity the 

 whole framework of this segmentation falls down. 



A successful division of the cavity that would stand criti- 



